In the mud logging field, one of the parameters of interest is the amount (i.e., concentration) of gas in the mud returning from the drilled well. There are different methods used to extract the gases from the mud. For example, mechanical agitators can be used in Gas Traps (included Constant Volume and Heated ones) to extract gas, or semipermeable membranes and other devices can be used. The extracted gases are then mixed with a carrier or transport gas (usually air or other convenient gas) and sent to analytical measurement device(s) that measures the gas concentration in the mixture with the carrier.
As one example, FIG. 1 shows a drilling system 10 that uses a gas trap (or degasser) and gas detection equipment to detect gasses in drilling mud leaving the flow line 16 from the wellbore. The drilling system 10 has a possum belly 20, a gas trap 50, a shaker 25, a mud pit 30, pumps 40, and other common components. In a typical installation, the gas trap 50 is fixed in the possum belly 20 from the flow line 16 at a fixed immersion level. As is known, the possum belly 20 is a container positioned at the head of the shale shaker 25 and is used to slow the drilling mud coming from the flow line 16 before passing over the shale shakers 25.
One example of a gas trap for use with such a drilling system is a Quantitative Gas Measurement (QGM) gas trap developed by Texaco and the Gas Research Institute in the 1990's. As background, the gas trap 50 shown in FIG. 2 can be similar to the device disclosed in U.S. Pat. No. 5,199,509. The trap 50 works as a centrifugal pump. A chamber 52 of the gas trap 50 is immersed in the drilling mud to an immersion level I. As a motor 60 spins an agitator 62 in the chamber 52, drilling mud is drawn up into the trap from a bottom inlet 54 and exits through a side pipe 55 immersed in the drilling mud. Air flow is brought in and out of the top of the trap 50 from a vent line 56 to a sample line 58. A pneumatic line links the gas trap's sample line 58 to the detection equipment (18), such as a chromatograph, IR total gas detector, or the like. Inside the chamber 52, a ring and baffle arrangement 64 can stabilize the mud circulation and increase the amount of time that the mud resides in the trap's container 52.
Historically, the first mud logging gas quantification procedures were done by comparison with a secondary extraction system (the so called steam or microwave still). An example of these procedures are detailed in QGM Quantitative Gas Measurement System by Texaco Inc. and Gas Research Institute (Chicago Ill.), Users Guide.
The above technology is used in connection with U.S. Pat. No. 5,199,509 that describes an improved Gas Trap, and with U.S. Pat. No. 5,447,052 that describes the Microwave Extraction of gases. Also, U.S. Pat. No. 8,011,238 describes quantification that uses Steam or Microwave still or comparison to PVT data.
One approach to quantify a gas extraction system uses a Constant Volume Trap (CVT). A constant flow of mud is sampled usually through a pump and sent to the gas extraction device (Gas Trap). After the pump, there may be a heater installed to raise the mud temperature to some higher values to improve the gas extraction. In this case, such a system is known as Heated Constant Volume Trap (HCVT).
For such systems, the quantification of the gas in mud is done on a certain volume of mud previously sampled in a separate container and then passed through the extraction system (CVT or HCVT) and collected in another container. Then, the mud collected is passed again through the extraction system and so on for a few cycle times. Mathematical functions are determined from the gas readings during the successive passes and are then used to quantify the original gas in mud amount. This kind of approach is described in U.S. Pat. No. 7,392,138, US 2011/0303463, and US 2014/0067307. The absolute gas in mud amount is an inferred value based on mathematical calculations and may be affected by the accuracy of the experimental, successive extraction runs.
Another approach can be used for gas extractions systems having a semipermeable membrane. In this approach, the semipermeable membrane is able to extract gases from the liquid mud based on the partial pressure difference of a gas outside the membrane (gas in mud) and inside the membrane. A carrier flow inside the membrane continuously swipes permeated gas and sends it to the analytics. In the meantime, the device keeps its partial pressure at low values to favor the permeation of gases from outside. The membrane is permeable only to gas and impermeable to liquid so this allows the device to be installed in a closed loop where a known amount of mud is circulated.
In a closed loop, amounts of gas of interest are injected. Then, the gas sampled through the membrane can be quantified to a true amount of gas in mud because of the known volumes of gas that were injected in a known mud volume. Such a volumetric injection quantification approach for semipermeable membranes is briefly described in SPE 62525 (June 2000). Yet, determining quantifications of each and every mud type in real-world implementations is difficult and time-consuming.
Finally, U.S. Pat. No. 5,648,603 discloses a way to quantitatively measure and detect gas entrapped in drilling fluid by injecting a known quantity of a standard gas into the evolved gas stream.
One problem with the various prior art approaches centers around the ability to translate the measured concentrations to actual concentrations of gas in mud because different gas types (different hydrocarbons) are extracted from the mud at a different rate due primarily to the different solubility of the gases in the mud. This is known in the technical literature as Trap Efficiency or Trap Factors or Solubility Correction Coefficients, Trap Calibration, or other names.
Determining the efficiency of gas extraction using conventional tools such as Gas Traps, Constant Volume Trap devices (including Heated), and others can be problematic. In fact, the actual gas extraction equipment is not calibrated as a system. Therefore, what is needed is a way to calibrate a gas extraction system as a whole.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.